1. Field of the Invention
The present invention concerns a monochromator for an X-ray device of the type having an X-ray source with a crystal for spectral restriction of the X-ray produced by the X-ray source. The invention also concerns an X-ray device that incorporates such a monochromator.
2. Description of the Prior Art
X-rays are used in medical and technical diagnostics to obtain images of objects to be examined. The quality of images thus produced depends on the radiation dose and on the energy spectrum of the X-rays. In order to achieve a certain minimum image quality, a certain minimum radiation dose is required, and the minimum radiation dose itself depends on the spectral energy distribution in the X-rays. In addition, depending on the concrete body or object to be examined, there always exists an optimum level of X-radiation energy, i.e., the wavelength of the X-radiation at which a maximum contrast resolution with a simultaneous minimized radiation dose can be achieved. Thus, in order to achieve the requisite minimum image quality with a minimized radiation dose, X-radiation of a suitable spectrum must be used.
The spectral energy distribution of X-rays, however, can be influenced at the X-ray source only to a limited extent. For example, the energy spectrum of a conventional X-ray tube always contains wavelength components outside the wavelength that is optimal for the radiation dose and the contrast resolution. The energy spectrum of an X-ray tube is influenced by the choice of anode material and by the type of X-ray absorption filters used. Furthermore, the aforementioned energy spectrum also strongly depends on the X-ray voltage, i.e., the energy with which electrons inside the X-ray tube are accelerated from the cathode to the anode. The X-ray voltage determines the upper limit of the energy spectrum.
Changes in the X-ray voltage affect not only the energy spectrum, but also the radiation dose, because with decreasing X-ray voltage, the tube current, i.e., the electron flow inside the X-ray tube, decreases. Thus, in order to compensate for the reduction of the radiation dose with a decrease in X-ray voltage, the X-ray tube current must be increased. The increase of the X-ray tube current, however, is restricted by the so-called blooming effect, by which—due to a lower X-ray voltage and high X-ray currents—the X-ray focal spot on the anode of the X-ray tube enlarges. The blooming effect negatively affects the properties of the X-rays that are produced.
Currently, depending on the particular application, a suitable energy spectrum is achieved by an appropriate combination of the anode material, the X-ray absorption filters, and the X-ray voltage. Each energy spectrum thus is necessarily a compromise among the various parameters.
European Application 0 924 967 discloses an X-ray device with a monochromator designed on the basis of a so-called mosaic crystal. The mosaic crystal is arranged in the path of radiation beam in such a manner that the X-rays of the X-ray tube are reflected by it. On the basis of Bragg relation for the diffraction of X-rays, for a certain reflection direction spectrally restricted, i.e., quasi-monochromatized, X-rays are obtained. In order to obtain X-rays of various wavelengths, the aforementioned European application proposes to implement multiple mosaic crystals to provide various Bragg angles. The arrangement of multiple mosaic crystals and their associated diaphragms requires a number of components and is therefore costly. Moreover, this arrangement has the inherent drawback that different propagation paths are pre-determined for the X-rays, which have to be individually aimed at the particular object to be examined.